Absorbent articles such as disposable diapers, incontinence pads, and catamenial napkins generally include an absorbent core for receiving and holding body exudates. The absorbent core typically includes a fibrous web, which can be a nonwoven, airlaid web of natural or synthetic fibers, or combinations thereof. A class of particulate absorbent materials known as superabsorbent polymers or absorbent gelling materials can be incorporated in the fibrous web to improve the absorption and retention characteristics of the fibrous web.
Because absorbent gelling materials are generally significantly more expensive than readily available natural or synthetic fiber materials (e.g., cellulose fibers), it is advantageous to reduce the quantity of absorbent gelling material in the core. Rather than uniformly reducing such particles throughout the entire core, it is desirable to distribute the particles in the absorbent core in a predetermined manner so that the particles are located where they will be most effective in acquiring and retaining body exudates.
Various techniques have been developed to distribute and locate absorbent materials on or within a fibrous substrate. U.S. Pat. No. 4,800,102 issued to Takada discloses applying a powder to the top surface of a substrate by spraying powder through an opening in a rotating disc. U.S. Pat. No. 4,028,224 issued to Pieper et al. discloses pulsing and diverting mechanisms for producing an intermittent flow of absorbent particles.
U.S. Pat. Nos. 4,543,274 issued Sep. 24, 1985 to Mulder discloses a powder spray gun wherein high velocity air is said to impact powder entrained air contained in the bore of the gun. U.S. Pat. No. 4,600,603 issued Jul. 14, 1986 to Mulder discloses a powder spray gun apparatus wherein an inverted flow amplifier is located adjacent to the inlet of the gun to enhance blending of powder within the gun. From the inverted flow amplifier, the blended powder is supplied to a downstream air flow amplifier which is operable to impact air entrained powder with a high velocity stream of compressed air. A powder control system controls powder supply from powder supply pumps to the spray gun. The powder pumps are said to be conventional venturi powder pumps.
U.S. Pat. No. 4,770,344 issued Sep. 13, 1988 to Kaiser discloses a powder spraying system including a volumetric or gravimetric material feeding device for metering a quantity of powder into a manifold, and air flow amplifiers connected to passageways formed in the manifold. Kaiser '344 teaches that a problem associated with venturi powder pumps is the difficulty in obtaining a consistently accurate feed rate of powder material, especially when a spray gun is operated intermittently. Applicants have also found that the use of venturi powder pumps and associated fluidized bed supply systems is undesirable because of difficulty in controlling powder feed rates, and because such systems can result in poor powder pulse definition. U.S. Pat. Nos. 4,927,346 and 5,017,324 issued to Kaiser disclose additional embodiments for depositing particulate material into a pad with a spray gun, including an embodiment having an inverted flow amplifier and an embodiment having a rotating screw for providing a metered quantity of absorbent particles.
U.S. Pat. No. 5,037,247 issued to Kaiser et al. discloses a powder pumping apparatus having a venturi passageway and an air nozzle including a valve mechanism. Kaiser '247 teaches that it is desirable to include a valve in the air nozzle to eliminate the "dead zone" in the air supply tube extending between the valve and the inlet to the pump body, and thereby eliminate the powder pulse "tailing effect" experienced in other powder pump designs. However, such an arrangement has the disadvantage of a requiring a valve assembly adjacent to or within the nozzle, which may not be practical or even possible in every installation due to space or geometry constraints.
While the above references provide descriptions of particle spraying systems suitable for use in forming absorbent webs, engineers and scientists continue the search for particle depositions systems that provide simple yet precise deposition of absorbent materials in fibrous webs.
Accordingly, it is an object of the present invention to provide an apparatus and method for applying discrete particles to a fibrous web.
It is another object of the present invention to provide a pulse of discrete particles for application to a predetermined location on a fibrous web.
Another object of the present invention is to provide an apparatus for applying powder to a fibrous web, the apparatus comprising a metering apparatus for providing a stream of powder having a predetermined mass flow rate, an acceleration air nozzle for providing a particle accelerating airflow, at least one deceleration air nozzle for providing a particle decelerating airflow, and valving for controlling airflow to the acceleration and deceleration air nozzles.